1. Field of the Invention
The present invention relates to a so-called coating-type magnetic recording medium having a magnetic layer which is formed by applying a coating material composed of a magnetic powder, a binder and a lubricant, onto a non-magnetic substrate.
2. Prior Art
Hitherto, so-called coating-type magnetic recording media produced by applying a coating material composed of a magnetic powder, a binder and a lubricant onto a non-magnetic substrate, have been widely utilized.
In this type of the magnetic recording media, it is general to use the binder composed mainly of vinyl-based resins such as vinyl chloride or vinyl acetate or copolymers thereof, cellulose-based resins, polyurethane-based resins, or the like. In general, these resins may be used singly or in the form of a mixture thereof in an optional combination depending upon aimed uses therefor.
For example, in the case where vinyl chloride-vinyl acetate copolymer which is a typical one of the vinyl-based resins is used, the binder exhibits a high elasticity and a good cross-linking ability so that a magnetic recording medium having a high durability and a good traveling performance can be obtained. However, the binder is unsatisfactory in dispersibility of the magnetic powder.
Similarly, in the case where nitrocellulose which is a typical one of the cellulose resins is used as the binder, it is impossible to absorb (admix) a high concentration of the magnetic powder therein, which is required to obtain a high dispersibility of the magnetic powder therein, because of its molecular structure. This leads to a limited application of the binder regardless of its high durability. In addition, acetyl cellulose can be used as the binder only in a limited amount from a standpoint of maintaining a good dispersibility of the magnetic powder.
Further, in the case where a polyester-polyurethane resin which is one of the polyurethane-based resins is used as the binder, various advantages such as good dispersibility of the magnetic powder, adherence to a base film as the non-magnetic substrate, etc., can be obtained. Nevertheless, the use of the polyester-polyurethane resin leads to inconveniences such as the production of the magnetic recording medium under limited conditions due to increase in viscosity of the coating material, deteriorated traveling performance and durability thereof due to increase in friction coefficient upon tape-traveling, or the like.
In order to obtain a magnetic recording medium having a high densification and a high performance, attempts have been made to further improve a fineness and a coercive force of the magnetic powder. In association with these improvements in the magnetic powder, there is an increasing demand for attaining a polymer resin which is suitable as a binder for such a magnetic powder and therefore can exhibit a high dispersibility to the magnetic powder together with a high durability or good traveling stability.
In order to attain useful binders, Japanese Patent Publication (Kokoku) No. Sho-58-41565 discloses, for example, a method in which functional groups acting for dispersing the magnetic powder is introduced into polymers for the binder. Also, there has been reported a method in which a silane-based coupling agent, a surface-modifying agent such as citric acid, etc., is added to the binder.
However, in these methods, properties or performance of the resulting magnetic recording medium are greatly influenced by the conditions of the surface of the magnetic powder and the effects obtained thereby are limited to particular combinations, so that it becomes difficult to attain magnetic recording medium having optimum properties.
As described above, conventional polymer resins are still unsatisfactory in providing a binder having good properties for the magnetic recording medium.
Consequently, in conventional magnetic recording media, a plurality of kinds of the polymers have been used in combination to attain a binder having improved properties. However, in such a combination of polymers, the use of an organic solvent is unavoidably required to prepare a coating solution dissolving the polymers, or particular combinations of the polymers must be selected to attain suitable physical properties such as compatibility therebetween. The method has also posed problems such as complicated procedures upon charging of the raw materials and the like. Under these circumstances, a further improvement in the binder or the like have been demanded to provide an excellent magnetic recording medium.